I believe it is correct that for a group, you must add the person as an e-mail address, i.e., you cannot simply add a cell phone number to a group for SMS purposes. Therefore you must use the subscriber's provider address rather than just a simple cellphone number.
And if you don't know the recipient's service provider, you can use
![[email address]](http://www.blackberryforums.com/?emailimage=ab78d8ba186343e4c44f24c6a05b8770)
, which converts to the providers address and sends the message.